This invention relates to apparatus for coupling microwave energy from a coaxial type transmission line to a double ridged waveguide transmission line and, more particularly, to a coax to waveguide transition which provides acceptable impedance matching between input and output transmission lines over a relatively broad bandwidth of microwave frequencies.
It is common practice in microwave transmission systems and in microwave devices, such as traveling wave tubes, to employ different types of microwave frequency transmission lines of known types, such as the coaxial type and the double ridged waveguide type, which have different electrical characteristics, such as characteristic impedance. It is also conventional in such systems to form a microwave energy transmission path for propagating microwave energy from one location to another in which the propagation path is formed of more than one kind of transmission line having different electrical transmission characteristics, such as the coaxial type and the double ridged waveguide type, a subject to which the present invention is directed. The coupling or connection between those two types of transmission lines is accomplished with reasonable efficiency by coupling means that are referred to by those skilled in the art as microwave transitions, or simply transitions.
Heretofore, many different physical structures for accomplishing a good electrical coupling between a coax and a waveguide line performing the function of a transition have been known, such as appears in the patents, believed to be exemplary of the prior art, U.S. Pat. Nos. 3,725,824, 3,737,812, 3,478,282, 3,528,041 and 3,431,515, which have been made known to us, to which the reader's attention is directed for additional background information. In its essentials, the output of the transition contains the configuration of a conventional waveguide type transmission line; the input of the transition contains the structure of the conventional coaxial type transmission line containing a central conductor surrounded by a dielectric; and intermediate the aforedescribed elements is employed an impedance matching section formed of tapered metal section or metal steps, physically resembling a staircase, or modifications and variations thereof.
The present invention is directed to a novel structure in a coax to waveguide transition of the recited type and has as a principal object the coupling of mircrowave energy over the frequency range of 8 to 18 gigahertz with reasonable levels of voltage standing wave ratios of between 1 and 2. An ancillary object of the present invention is to define a physical structure of a transition which is easy to fabricate and adjust.